The present invention relates to receiving apparatuses for satellite broadcasting. More specifically, the invention relates to receiving apparatuses having a capability of receiving a plurality of video signals and outputting other signals to a plurality of tuners.
A radio wave transmitted from a satellite includes at least one polarized wave, and each polarized wave has a video signal. For example, there are several types of polarized waves such as a vertically polarized wave and a horizontally polarized wave (Vxe2x80xa2H) and a right-hand circularly polarized wave and a left-hand circularly polarized wave (Rxe2x80xa2L).
Among receiving apparatuses for satellite broadcasting which receive radio waves transmitted from a satellite, there is one type of receiving apparatus for satellite broadcasting with one Low Noise block downconverter (hereinafter referred to as LNB) which receives signals transmitted from a plurality of satellites. Also, there is another type of receiving apparatus for satellite broadcasting with an LNB which extracts a plurality of IF signals (video signal) by utilizing two or more local oscillating circuits having respectively different oscillation frequencies. Currently, the dominating receiving apparatus for satellite broadcasting is the type of apparatus with one LNB which extracts a plurality of video signals (two kinds, four kinds, or more of video signals).
FIG. 1 shows an example of a circuit configuration of the type of LNB which outputs a plurality of IF signals.
The LNB in the example mainly includes the following circuits parts.
(1) Low Noise Amplifiers (hereinafter referred to as LNAs) 1a and 1b: Amplify weak signals (of 10 GHz band) transmitted from satellites in low noise characteristic.
(2) Band Pass Filters (hereinafter referred to as BPFs) 2a and 2b: Band-limit the signals amplified in low noise characteristics.
(3) Local oscillating circuits 4a and 4b: Produce a reference frequency (local signal) for downconverting.
(4) Mixer circuits 3a and 3b: Mix the signals amplified in the LNA with the local signal, and convert (downconvert) the resultant signals into IF signals (of 1 GHz band).
(5) Switch circuit 5: Extracts the IF signal selected by a tuner (not shown), which is connected to output terminals 7a and 7b, from among the signals converted in the mixer circuits 3a and 3b, and the extracted IF signal is sent to the IF amplifiers 6a and 6b. 
(6) IF amplifiers 6a and 6b: Amplify the IF signal extracted in the switch circuit 5 to output it to the output terminals 7a and 7b. 
Incidentally, the receiving apparatus that receives radio waves transmitted from satellites has the problem of isolation characteristic. The isolation characteristic is a capability of preventing the entrance of undesired signals as opposed to desired signals. In the receiving apparatus, it is ideal that undesired signals not being selected never leak out. However, in fact, undesired signals in the range of approximately 20 to 30 dB enter into the terminal to which desired signals should be inputted, because of problems caused by the circuit configuration and the member arrangement in the receiving apparatus.
In order to obtain a better isolation characteristic of the receiving apparatus, it is necessary not only to make the isolation characteristic of the switch circuit 5 better, but also to prevent the video signals after being amplified by the IF amplifiers 6a and 6b from entering into the respective output terminals 7b and 7a that are not appropriate terminals to which the IF amplifiers 6a and 6b should be inputted. However, a conventional receiving apparatus has found a difficulty of achieving this. The reason of the difficulty will be described below.
First, to maintain hermeticity, a receiving apparatus must be generally hermetic in a space where a circuit substrate is included. Conventional methods for hermetically sealing are the following methods: one is the method in which a metallic lid (hereinafter referred to as a back lid 8) is fixed to a chassis 10 with a packing 9, as shown in FIG. 2 (in FIG. 2, note that, a reference numeral 10b is a groove where the packing is placed); or another is the method in which while a bottom part of the back lid 8 is fit into a groove 10c, a seal adhesive such as silicon is filled so that the back lid 8 is fixed to the chassis 10, as shown in FIG. 3. These methods for hermetically sealing cause an electrically poor connection between the back lid 8 and the chassis 10. Accordingly, in these methods for hermetically sealing, the back lid 8 is insufficiently grounded in a radio-frequency band, so that the receiving apparatus is prone to the occurrence of resonance.
Because of this, in the conventional receiving apparatus, there is a high possibility that factors such as resonance cause the video signals selected by the tuner which is connected to one output terminal to enter into the other output terminal, when a back lid that is grounded insufficiently is fit to the receiving apparatus. Therefore, even if the receiving apparatus without the back lid realizes a high isolation characteristic (of, for example, 40 dB or more), the receiving apparatus with the back lid is difficult to realize a high isolation characteristic.
Here, as a technique for shielding a receiving apparatus, Japanese Unexamined Patent Application No. 308219/1993 (Tokukaihei 5-308219, published on Nov. 19, 1993) discloses a microwave detector which realizes a minimum length of connecting path between the mixing diode and an intermediate frequency processing circuit portion, as well as excellent operation with avoiding generation of the noise without a first intermediate frequency filter being affected by an antenna input and/or the output of a first local oscillator. The microwave detector has such a structure that the intermediate frequency processing circuit is installed around an extended portion of the mixing diode on the lower surface of a printed circuit board, that a shield casing is mounted directly on the lower surface side of the printed circuit board in such a manner that it may cover the intermediate frequency processing circuit portion, and that a metal layer is formed on the upper surface of the printed circuit board, thereby completely shielding the intermediate frequency processing circuit.
Also, Japanese Unexamined Patent Application No. 38827/1995 (Tokukaihei 7-38827, published on Feb. 7, 1995) discloses an IF substrate incorporating tuner. To prevent the leakage of spurious radiation to the IF substrate, the IF substrate incorporating tuner has such a structure that a tuner substrate and an IF substrate are shielded from each other by a shield plate, and the tuner substrate and the IF substrate are connected via through capacitor provided on the shield plate. The feature of the invention is that the through capacitor as a connecter section makes it possible to almost completely shield the tuner substrate, thereby preventing the leakage of oscillator to the IF substrate and the deterioration of the radiation.
Furthermore, Japanese Unexamined Patent Application No. 163755/1999 (Tokukaihei 11-163755, published on Jun. 18, 1999) discloses an up/down tuner. To reduce a local oscillator circuits of up converter and a down-converter affecting each other, and inflow noise that comes into the tuner from the outside, the up/down tuner has a structure that an up converter part and a down converter part are connected to each other by an IF connecting part which makes a coaxial shielded structure (a structure that a signal line is shielded by an electrically conductive material grounded).
However, the techniques described in the above patent applications relates to reduction of the noise from other circuits, the leakage of spurious radiation, and inflow noise that comes from the outside, in other words, relates to noise-resistant characteristic. That is, the techniques described in the above patent applications do not relate to improvement of isolation characteristic, or a capability of preventing the entrance of undesired signals as opposed to desired signals. Further, the above patent applications do not suggest the increase in the isolation characteristic.
The present invention has been developed to solve the above problem, and an object of the present invention is to provide a receiving apparatus which is capable of realizing a high isolation characteristic of, for example, 40 dB or more.
In order to achieve the above object, a receiving apparatus according to the present invention includes a circuit substrate having IF amplifiers and output terminals connected to tuners, in which a plurality of different radio waves such as polarized waves and satellite waves are received, and a plurality of video signals are extracted from radio waves thus received and are amplified so as to be outputted to the tuners. The receiving apparatus further includes a shield member made of metal (for example, a metallic plate such as stainless steel or phosphor bronze) for obtaining a high isolation characteristic. More specifically, the shield plate is mounted above the IF amplifiers and output terminal mounting parts included in the circuit substrate.
Inside the receiving apparatus, the level of signals is highest in the IF amplifiers and output terminal mounting parts. In this regard, according to the above arrangement, the shield member which is made of metal is mounted above the IF amplifiers and the output terminal mounting parts of the circuit substrate. Because of this, the shield member makes it possible to decrease adverse effects caused by the back lid with unstable earthing.
That is, due to the occurrence of resonance, the inflow of undesired signals increases in a space which is produced by the mounting of the back lid with unstable earthing. In this regard, in the present invention, provision of the shield member having stable earthing inside the space causes change of the condition where signals are reflected in the space, thereby preventing the occurrence of resonance. This results in the improvement of isolation characteristic (for example, high isolation characteristic of 40 dB or more).
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.